This invention relates to boiler systems in general and more particularly to a control system for controlling the operation of a series of boilers to achieve maximum efficiency of operation.
Modern day boilers are designed for efficient operation and as such include modulation apparatus. Essentially, the term modulation means to adjust by increments and decrements or to modify by varying a second condition the firing rate position of a boiler. In this manner, the firing rate of the boiler is under the control of an external control panel or control apparatus. In order to provide modulation, boilers employ a flame safeguard control that provides an infinite number of firing rate positions to allow the boiler to fire in response to demand. The modulating controller is a device which automatically positions the firing rate of the boiler between its extreme positions in response to system demands. Thus many manufacturers supply boilers which incorporate modulation controllers as described. See for example a brochure entitled M-SERIES GAS AND OIL BURNERS by IC, Industrial Combustion of Milwaukee, Wis. This brochure describes a full modulating burner for a boiler as well as high-low modulating burners.
These boilers in automatic operation employ a sequential operating cycle which operates the burner and the boiler through pre-purge, pilot ignition, main flame ignition, run and post purge. The length of purge and ignition trial vary according to the type of programmer controller employed. During the run cycle, burner output is regulated to the load demand by the modulating pressure or temperature control on the boiler. The burner will continue to modulate until the operating pressure or temperature is reached.
Thus many manufacturers supply programming controls to allow modulating operation of boilers. See for example a brochure entitled FLAME SAFEGUARD PROGRAMMING CONTROLS (R4140L) by Honeywell of Minneapolis, Minn. (1979). This describes programmers or controllers which provide flameout protection plus automatic sequencing of the burner motor (blower), firing rate motor, ignition, pilot value and main fuel valves for commercial and industrial burners on boiler using coal, gas, oil or a combination of fuels.
In many modern facilities, boilers are used in parallel. A modern building or plant may use at least two or more boilers which are arranged in parallel and are operated to supply the peak and normal demands of a facility. As such there is a lead boiler and at least one lag boiler in such systems. The lead boiler is used as the main system boiler with the lag boiler employed as a back-up device when the lead boiler fails. This type of operation as will be explained is very inefficient and results in many problems in that the full modulating capabilities of the boilers are not employed and thus results in shortened boiler life due to the extended use of the lead boiler as controlled by certain prior art systems techniques.
Cognizant of such problems, certain manufacturers attempted to solve these problems by providing Lead-Lag programming controllers. See a brochure entitled CHIEF DISPATCHER by Preferred Instruments of Danbury, Conn. (1972) BULLETIN SDI-JD-DCBF(A). This describes a lead-lag programming control to integrate the multiple boiler installation into a coordinated system. Thus the controller automatically sequences the firing of several boilers in balance with changing load conditions. The unit programs the individual boilers in or out of operation in response to predetermined pressure or temperature variations. This is done to equalize equipment usage rate. Hence the user can select any boiler as the lead boiler by rotating an external sequence selector switch and lead boiler can be changed as often as desired. However, this has to be done manually and this operation is often neglected. Thus the same company provides an alternative model. See BULLETIN SDI-JC-CDDF(A) (March, 1972) where the system described automatically alternates the lead boiler with each call for operation. For example, on the first call the No. 1 boiler will fire as the lead boiler. On the next call the No. 2 boiler will fire as the lead boiler and so on. If a lead boiler fails, another boiler will take its place.
However, this prior art approach is attendant with many problems due to the fact that if there is a failure of the lead boiler, the next lag boiler, which is the next boiler in line, will come on only after the pressure has dropped to the lag boiler setting. By this time, the system has lost pressure and energy and the lag boiler due to system operation will not meet the lead boiler's load. This is because the operating and modulation controls of each boiler does not change when a new lead boiler is selected. Hence a former lag boiler when selected as the lead boiler still has the lag boilers modulation and operating control rate and therefore the lag boiler selected cannot operate to meet the lead boiler's load requirements.
It is, thereofore, an object of the present invention to provide an improved boiler control apparatus whereby the operating control and modulation of a boiler is changed when that boiler is selected as a lead boiler to thereby achieve increased efficiency and prolong boiler life.